The present invention relates to a magnetic resonance imaging (MRI) system which generates a magnetic resonance (MR) in an object to be examined, detects an MR signal in accordance with the MR, and performs processing including image reconstruction processing, the image reconstruction processing being established by computed tomography, thereby obtaining a distribution image of MR data in a selected slice in the object and, more particularly, to an improvement of a reception probe for detecting an MR signal excited in an object to be examined in an MRI system mainly used in medical examination.
In an MRI system, an object to be examined is placed in a uniform static field, and an inclined magnetic field is superposed on the uniform static field and an excitation rotating field is applied thereto, thereby causing an MR phenomenon in a prospective slice portion of the object. An MR signal generated by the MR phenomenon is detected, and is subjected to predetermined processing including image reconstruction processing, thereby obtaining a slice image to which MR data in a slice of the object is reflected. As the MRI system, a medical diagnosis system is widely known.
In MRI examination, an appropriate combination of magnetic fields, which are generated by a magnet device including coils, is applied to an object to be examined. An MR excited upon application of the combination of magnetic fields is detected by at least one reception coil system. An MR signal detected by the reception coil system is subjected to predetermined processing in order to obtain MR image data.
An imaging operation in an MRI system will be schematically described with reference to FIG. 1 showing a conventional MRI diagnosis system. The object to be examined (patient) P is placed in satisfactorily uniform static field H0 along a Z axis in FIG. 1, which is generated by a static field generator (not shown). Gradient field Gz along the Z axis in FIG. 1 is added to static field H0 by a pair of inclined magnetic field coils llA and llB. These Z axis inclined magnetic field coils llA and llB are formed by, e.g., a pair of Helmholtz coils. A magnetic field intensity distribution in which a magnetic field intensity differs in accordance with displacement along the Z axis is obtained by gradient field Gz along the Z axis. An RF rotating field of angular frequency .omega.0 which can resonate specific nuclei, i.e., excitation pulse H1, is applied to patient P by a transmission probe. The transmission probe comprises a pair of transmission coils 12A and 12B arranged in, e.g., a probe head. These coils 12A and 12B comprise a pair of saddle coils, a shown in FIG. 1. RF excitation pulse Hl is applied to patient P through coils 12A and 12B. When pulse Hl is applied, an MR phenomenon occurs only in slice portion S (which is a planar portion, but has a given thickness in practice) perpendicular to the Z axis selectively determined along the Z axis by gradient field Gz. The MR phenomenon is detected as an MR signal by a reception probe. The reception probe comprises reception coils 13A and 13B arranged in, e.g., the probe head. The MR signal detected by coils 13A and 13B is a free induction delay (FID) signal or a spin echo signal. The MR signal (the FID or spin echo signal) detected by coils 13A and 13B depends on an MR excitation method (excitation pulse sequence). In order to obtain a slice image consisting of MR data in slice portion S, two-dimensional position data on the plane of slice portion S is necessary. For this reason, in order to add the two-dimensional position data to the MR signal, after slice portion S is excited to cause the MR phenomenon, gradient field Gxy having gradients in various directions on the X-Y plane is applied to field H0 by coils (not shown). The thus detected MR signal is subjected to predetermined processing, e.g., two-dimensional Fourier transform, thereby reconstructing a slice image consisting of the MR data in slice portion S.
In the conventional MRI system, the probe head is arranged inside coils for generating static and gradient fields, and serves as a transmission/reception probe. A pair of saddle coils (serving both as transmission and reception coils) are arranged in the probe head to surround a patient.
The probe head applies an RF field for exciting an MR phenomenon to a patient, as a transmission probe, and detects a weak MR signal from the patient, as a reception probe. For this reason, the probe head is preferably arranged as near the patient as possible for higher efficiency.
In the conventional probe head, a pair of saddle coils 22A and 22B are arranged on cylindrical bobbin 21, as shown in FIG. 2. Saddle coils 22A and 22B in probe head 23 are connected to each other, as shown in FIG. 2. Probe head 23 is used such that the abdomen of patient P is placed inside head 23, as shown in FIG. 3.
However, with probe head 23 having the above arrangement, the space between head 23 and patient P must be widened so that patient P lying on a bed for diagnosis does not experience anxiety or feel cramped. For this reason, since the distance between patient P and head 23 becomes large, head 23 does not always have a best shape as a reception probe for receiving, in particular, a MR signal which is usually weak.
Since conventional head 23 has a cylindrical shape, coils generating static and gradient fields are arranged inside a gantry incorporated therein, and patient P is inserted and lies therein to have a predetermined positional relationship with head 23. For this reason, an operation for setting patient P to obtain the predetermined positional relationship with head 23 is very cumbersome.
Since bobbin 21 is normally formed of plastic as a dielectric, loss occurs upon reception of a weak RF signal, i.e., an MR signal.